1. Field of the Invention
The present invention relates to micro-mechanical modulators that are used to modulate light signals propagated by optical fibers. More particularly, the present invention relates to surface normal mechanical optical modulators and the structure of the membranes used by such modulators.
2. Description of the Prior Art
In many optical wavelength division multiplexing systems it is desirable to have inexpensive optical modulators that have high contrast and wide optical bandwidths. One such modulator is a surface normal micro-mechanical modulator, sometimes referred to as a mechanical antireflection switch (MARS) modulator. Such modulators have a variable air gap defined between a substrate and a membrane. Typically, MARS modulators operate by selectively changing the amount of light reflected in the surface normal direction, i.e., the direction normal to the substrate below the membrane. This is done by varying the size of the air gap that exists in between the substrate and the membrane, thereby altering the optical properties of the device.
Since the optically active area of the membrane is not coated with a metal film, an electric charge can build upon the dielectric silicon nitride film in this area due to surface and bulk migration of charges. The build up of charges in the optically active area of the membrane changes the required voltage needed to accurately operate the MARS modulator. Inaccurate operation of the MARS modulator can adversely affect operation of the entire optical network.
A need, therefore, exists for an improved MARS modulator where surface charges are prevented from collecting on the optically active area of its membrane.